给定一个二叉树，返回它的 后序 遍历。
示例:
代码语言：javascript复制输入: [1,null,2,3]
1
2
/
3
输出: [3,2,1]进阶: 递归算法很简单，你可以通过迭代算法完成吗？
解：送分题咯。
代码语言：javascript复制/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
import javafx.util.Pair;
class Solution {
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
Stack<Pair<TreeNode, Boolean>> stack = new Stack<>();
stack.push(new Pair<>(root, false));
while (!stack.isEmpty()) {
TreeNode node = stack.peek().getKey();
boolean visited = stack.peek().getValue();
stack.pop();
if (node == null) {
continue;
}
if (visited) {
res.add(node.val);
} else {
stack.push(new Pair<>(node, true));
stack.push(new Pair<>(node.right, false));
stack.push(new Pair<>(node.left, false));
}
}
return res;
}
}
